Field of the Invention
The present invention relates to helicopter control systems and more particularly to an automatic pitch change mechanism for rotary wing aircraft with a simplified control system.
Discussion of the Related Art
A helicopter is a flight vehicle that derives its lift and thrust from one or more rotating sets of airfoils called rotors. In a most common configuration, a helicopter will have a single main rotor having two or more blades rotating primarily in a horizontal plane about a vertical shaft above the fuselage of the vehicle. The rotating main rotor causes a counter rotational force on the fuselage which must be counteracted to maintain the fuselage in a stable position. Most helicopters also have a smaller tail rotor at the rear of the fuselage to counter the rotational forces placed on the fuselage by the main rotor, other helicopters utilize fans or ducted airflow at the tail of the helicopter to provide the necessary forces to counter this rotational force.
The helicopter main rotor is powered by the engine, through a transmission to a rotating mast. The mast is a cylindrical metal shaft extending upward from the transmission. At the top of the mast is mounted a hub to which two or more rotor blades are attached. Main rotor systems are classified according to how the main rotor blades are attached and move relative to the main rotor hub. Main rotor systems fail into one of three basic configurations: rigid, semi-rigid, or articulated although some rotor systems use an engineered combination of these classifications.
Maneuvering of the helicopter is primarily accomplished through pilot inputs to the main rotor through the use of collective and cyclic controls which impart adjustments to the main rotor to provide the desired directional movements of the helicopter. Foot pedals operable by the pilot allow for yaw control of the fuselage about the axis of the mast.
An articulated rotor system in one where each rotor blade is attached to the rotor hub through a series of hinges allowing the blade to move independently of the others. These rotor systems usually have three or more blades. The blades are allowed to flap, feather, and lead or lag independently of each other. A horizontal hinge, called the flapping hinge allows the blade to move up and down. This movement is called flapping and is designed to compensate for dissymmetry of lift. The flapping hinge may be located at varying distances from the rotor hub, and there my be more than one hinge. The vertical hinge, called the lead-lag or drag hinge, allows the blade to move back and forth. This lead-lag or dragging movement compensates for the acceleration and deceleration caused by momentum conservation. Excessive back and for the movement around the drag hinge is usually prevented through use of dampers.
A semi-rigid rotor is normally composed of two blades which meet just under a common flapping or teetering hinge at the rotor shaft. This allows the blades to flap together in opposite motions like a seesaw. This underslinging of the blades below the teetering hinge, combined with an adequate dihedral or coning angle on the blades, minimizes variations in the radius of each blade's center of mass from the axis of rotation as the rotor turns, which in turn reduces the stress on the blades from lead and lag forces. Secondary flapping hinges may also be provided to permit sufficient flexibility to minimize bouncing.
Articulated and semi-rigid systems also typically include a feathering hinge to permit changes to the pitch angle of the blade. The pitch angles of the blades can be changed uniformly with a single control input from the collective control. The pitch angles of the blades can also be changed according to the position of the blade in the cycle of the rotor disk, increasing and decreasing as the blade rotates about the mast. Collective and cyclic pitch changes to the rotor blades are accomplished by adjusting the orientation of a swash plate interconnected to the individual rotors.
The swashplate in the helicopter control stream has a pair of plates, one rotating and one fixed, that are centered on the main rotor shaft. The rotating plate is linked to the rotor head, and the fixed plate is linked to the operator cycle and collective controls. Angular displacement of the alignment of the fixed plate is transferred to the rotating plate, where it becomes reciprocal motion of the rotor blade linkages. This type of pitch control, known as cyclic pitch, allows the helicopter rotor to provide selective lift in any direction. Axial displacement of the swash plate along the axis of the mast translates into a uniform movement of the rotor blade linkages about the rotor hub. This type of pitch control, is known at collective pitch, allows the helicopter rotor to provide uniform lift along the mast axis.
A rigid rotor system usually refers to a hingeless rotor system with blades that are flexibly attached to the hub. In a rigid rotor system, each blade flaps and drags about flexible sections of the blade root. A rigid rotor system is mechanically simpler than a fully articulated rotor system. Loads from flapping and lead/lag forces are accommodated through rotor blade flexure, rather than through hinges. By flexing, the blades themselves compensate for the forces which previously required rugged hinges. This results in a rotor system that has less lag in the control response because the rotor has much less oscillation. The rigid rotor system also negates the danger of mast bumping inherent in teetering rotors found in semi-rigid systems.
Two bladed rotor systems can also include a stabilizer bar or flybar oriented perpendicular to the axes of the two diametrically opposed rotor blades. The stabilizer bar has a weight or paddle at either end which cause the bar to stay relatively stable in the plane of rotation and reduces crosswind thrust on the rotors. Through mechanical linkages, the stable rotation of the bar is mixed with the swashplate movement so that internal as well as external forces on the rotor are damped. This eases the workload of the pilot to maintain control of the helicopter.
Previous and existing helicopter rotor systems depend on mechanically complex mechanisms to control each rotor blade for its proper lift vector. Blade pitch is varied as it rotates into and away from the relative air velocity vector while in forward flight through use of cyclic pitch controls. These controls make fully articulated rotor systems difficult to control and especially as a remotely piloted vehicle. The mechanical complexity also contributes to considerably expensive design, construction, and maintenance.
Thus what is desired is an articulated helicopter rotor system that is mechanically simple and easy to control.